Today there is a wide range of wind turbines comprising a tower that consists of a lattice structure either partially or in its entirety. There is extensive prior art in relation to connecting lattice towers and the final tower segments that will support the nacelle. The element that is most commonly used as the transition piece is a disc that can have different configurations and even different materials, depending on the use to be made of it.
In that sense, patent document US 2012023860 A1 discloses the transition between a lattice tower and a tower segment supporting the nacelle. The transition is done with a concrete disc such that the metallic ends of the lattice tower are embedded in the solid disc formed by concrete.
Patent document US 20090249707 A1 discloses the transition through a circular ring separating the fixed part corresponding to the lattice tower from the moving part or nacelle. The ring is metallic, not very thick compared to its diameter and it is connected to the yaw ring which in turn supports the nacelle by means of four bearings.
However, this latter solution is not altogether satisfactory because the ring has a large diameter and it is not very thick, and it cannot withstand the stresses required for a large sized nacelle. In addition, the cost of the material and the transport and mounting of a ring of these characteristics is not viable.
The size of wind turbines has increased over time, and accordingly the momentums and stresses to be withstood in the structure housing the nacelle have increased. The transition between the tower and the nacelle must be provided by a structural piece that transmits the aerodynamic and gravitational loads of the wind turbine from the rotor to the tower, therefore passing through the nacelle and through the transition piece between the yaw system thereof and the tower. The resulting vertical loads, which are transverse to the plane of the ring provided in patent document US 20090249707 A1, generate significant bending therein. Since the ring is not very thick, it does not present sufficient flexural rigidity. In order to withstand said bending loads, the lattice of the tower must branch out and the distance between supports of said ring must be reduced. As a result, a very complex three-dimensional structure is obtained in the transition area.
This and other problems are solved in the apparatus disclosed herein, where the transition structure presents enormous flexural rigidity and allows direct transition between the continuous crown of the yaw system of the nacelle and the vertical members of the lattice tower.
There are a number of lattice tower configurations in which the number of main columns and the distance between columns along the height of the tower vary. The most extreme case as regards complexity for the solution of the transition piece is that of a tower having three main columns and a constant distance between columns along the entire height thereof. In this case, the transition piece may have a diameter close to 15 meters and the distance between columns may be about 12 meters.
The transition structure disclosed herein can withstand said loads. Furthermore, a larger number of legs or an additional support between legs can be added to increase support points in the plane of contact between the transition piece and the lattice.
Obtaining this transition piece which, attached to a lattice tower and a nacelle, withstands all the stresses to which it is subjected, which can include a yaw system for the nacelle and which provides stable support with a certain maneuverability for mounting the elements integrated in the nacelle, is a huge design challenge, and the object of the present invention is to provide a low-weight structure that efficiently transmits loads and has a low associated cost. This solution will also be efficient with smaller diameters and a larger number of main columns.